Techniques for improving channel estimation and tracking in a wireless communication system

ABSTRACT

A technique for performing channel tracking and/or channel estimation in a wireless communication device includes receiving a reference signal and one or more non-error propagation physical channel signals. In general, the one or more non-error propagation physical channel signals must be correctly decoded before a data channel can be decoded. Channel tracking and/or channel estimation are/is then performed based on the reference signal and at least one of the one or more non-error propagation physical channel signals.

BACKGROUND

1. Field

This disclosure relates generally to channel estimation and, morespecifically, to techniques for improving channel estimation andtracking in a wireless communication system.

2. Related Art

As is well known, a wireless channel provides an arbitrary timedispersion, attenuation, and phase shift in a transmitted signal. Whilethe implementation of orthogonal frequency division multiplexing (OFDM)with a cyclic prefix in a wireless communication system mitigates theeffect of time dispersion caused by a wireless channel, in order toapply linear modulation schemes it is also usually necessary to removeamplitude and phase shift caused by the wireless channel. Channelestimation is typically implemented in a wireless communication systemto provide an estimate (from available pilot information) of anamplitude and phase shift caused by a wireless channel. Equalization maythen be employed in the wireless communication system to remove theeffect of the wireless channel and facilitate subsequent symboldemodulation. Channel tracking is usually employed to periodicallyupdate an initial channel estimation. For example, channel tracking maybe employed to facilitate periodic frequency-domain and time-domainchannel correlation and periodic updating of channel signal-to-noiseratio (SNR), channel delay spread, and channel Doppler effect.

In a known communication system, a channel coefficient is estimatedbased on a training signal (pilot) and data is decoded using anestimated channel coefficient. In this system, data is subtracted from areceived signal (to reduce interference with the training signal) andthe channel estimation is updated based on y(n)−x̂ (n), which can berepeated for a number of iterations (where y(n) corresponds to thereceived signal and x̂(n) corresponds to a reconstructed data signal). Inanother known communication system, a transmission channel state isobtained from a difference between a received signal and a hardquantized value of a detected signal, which is obtained through anequalizer based on an interpolated channel estimate from scatteredreference signals (pilot signals). Unfortunately, in the knowncommunication systems, errors may be propagated as a reconstructed datasignal, which is subtracted from a received signal to provide a channelestimation, may be inaccurate.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example and is notlimited by the accompanying figures, in which like references indicatesimilar elements. Elements in the figures are illustrated for simplicityand clarity and have not necessarily been drawn to scale.

FIG. I is a diagram of an example downlink (DL) frame transmitted from aserving base station (BS) in a long-term evolution (LTE) compliantwireless communication system.

FIG. 2 is an example diagram of a prior art approach for performingchannel estimation.

FIG. 3 is an example diagram of an approach for performing channelestimation/tracking according to an embodiment of the present invention.

FIG. 4 is a flowchart of an example process for performing channelestimation/tracking according to one aspect of the present invention.

FIG. 5 is a block diagram of an example wireless communication systemthat includes wireless communication devices that may perform channelestimation/tracking according to various embodiments of the presentinvention.

DETAILED DESCRIPTION

In the following detailed description of exemplary embodiments of theinvention, specific exemplary embodiments in which the invention may bepracticed are described in sufficient detail to enable those skilled inthe art to practice the invention, and it is to be understood that otherembodiments may be utilized and that logical, architectural,programmatic, mechanical, electrical and other changes may be madewithout departing from the spirit or scope of the present invention. Thefollowing detailed description is, therefore, not to be taken in alimiting sense, and the scope of the present invention is defined onlyby the appended claims and their equivalents. In particular, althoughthe preferred embodiment is described below in conjunction with asubscriber station, such as a cellular handset, it will be appreciatedthat the present invention is not so limited and may potentially beembodied in various wireless communication devices.

As used herein, the term “channel” includes one or more subcarriers,which may be adjacent or distributed across a frequency band. Moreover,the term “channel” may include an entire system bandwidth or a portionof the entire system bandwidth. As used herein, the term “referencesignal” is synonymous with the term “pilot signal.” As is also usedherein, the term “subscriber station” is synonymous with the term “userequipment,” which includes a wireless communication device that may (ormay not) be mobile. A reference signal (RS), when received at asubscriber station (SS), is utilized by the SS to perform channelestimation and channel tracking. The disclosed techniques arecontemplated to be applicable to systems that employ a wide variety ofsignaling techniques, e.g., orthogonal frequency division multiplex(OFDM) signaling, single-carrier frequency division multiple access(SC-FDMA) signaling, etc.

In general, accurate channel estimation and tracking is desirable toachieve acceptable performance for SSs in a wireless communicationsystem (e.g., a long-term evolution (LTE) wireless communication system)as downlink (DL) performance is determined by the accuracy of channelestimation and channel tracking. According to various aspects of thepresent disclosure, one or more non-error propagation physical channelsare employed in conjunction with a DL reference signal (DLRS) to improvethe accuracy of channel estimation and tracking. In this manner,performance (e.g., block error rate) of a receiver (e.g., an LTEreceiver) may be improved. In LTE, an RS is distributed in a subframeand interpolation is used to perform channel estimation for an entiretime-frequency grid of an OFDM signal. Unfortunately, employinginterpolation may introduce errors in channel estimation.

In the case of an LTE system having a 1.4 MHz system bandwidth, onlytwelve DLRS subcarriers are currently allocated (in a first symbol ofeach subframe) for channel estimation and tracking. According to variousaspects of the present disclosure, a number of non-error propagationphysical channels may be utilized in conjunction with a DLRS to improvechannel estimation and tracking. For example, in an LTE compliantsystem, the non-error propagation physical channels include: asynchronization channel (SCH), which includes a primary synchronizationchannel (PSCH) and a secondary synchronization channel (SSCH); a primarybroadcast channel (PBCH); and a physical control format indicationchannel (PCFICH). It should be appreciated that while the discussionherein is directed to an LTE system, the techniques disclosed herein arebroadly applicable to improving channel estimation and tracking in anywireless communication system that employs one or more non-errorpropagation physical channels. As used herein, the term ‘non-errorpropagation physical channel’ is a channel that is utilized to transmitcall set-up information to an SS. The call set-up channels are generallyphysical layer control message channels that include DL and uplink (UL)assignments for SSs in a cell. Broadly, non-error propagation physicalchannels may be thought of as channels that do not include SS data andmust be correct for an SS to receive data.

As noted above, in an LTE system, the DLRS is designed for channelestimation and tracking. In general, detection of the SCH (which istypically the most reliable of the physical channels in an LTE compliantsystem) does not rely on channel estimation. In an LTE compliant system,the PBCH is usually an extremely reliable channel that typically employsa coding rate less than 1/48. According to various aspects of thepresent disclosure, the PBCH is only utilized for channel estimation andtracking when the PBCH passes a cyclic redundancy cheek (CRC). In an LTEcompliant system, the PCFICH is usually a very reliable channel thattypically employs a coding rate of 1/16. In general, if PCFICH fails, anassociated subframe fails and, as such, performing channel estimationwith a failed PCFICH will not usually worsen receiver performance. In atypical LTE compliant system, the SCH can provide 62×2 additionaltraining pilots every 5 milliseconds, the PBCH can provide 72×4additional training pilots every 10 milliseconds, and PCFICH can provide16 more training pilots every 1 milliseconds. According to the presentdisclosure, one or more of the SCH, PBCH, and PCFICH may be utilized astraining pilots (in conjunction with a DLRS) to improve channelestimation/tracking, as the SCH, PBCH, and PCFICH are implemented insuch a way that feedback does not cause error propagation.

According to one embodiment of the present disclosure, a technique forperforming channel estimation in a wireless communication deviceincludes receiving a reference signal and one or more non-errorpropagation physical channel signals. Channel estimation is thenperformed based on the reference signal and at least one of the one ormore non-error propagation physical channel signals.

According to another embodiment of the present disclosure, a wirelesscommunication device includes a receiver (e.g., included in atransceiver) and a processor (e.g., a digital signal processor (DSP))that is coupled to the receiver. The receiver is configured to receive areference signal and one or more non-error propagation physical channelsignals. The processor is coupled to the receiver and is configured toperform channel estimation based on the reference signal and at leastone of the one or more non-error propagation physical channel signals.In an LTE compliant system, the one or more non-error propagationphysical channel signals include a synchronization channel signal, aprimary broadcast channel signal, and a physical control formatindication channel signal, one or more of which may be utilized inconjunction with the reference signal to improve channel estimation andtracking.

According to yet another embodiment of the present disclosure, atechnique for performing channel tracking in a wireless communicationdevice includes receiving a reference signal and multiple non-errorpropagation physical channel signals. Channel tracking is then performedbased on the reference signal and at least one of the multiple non-errorpropagation physical channel signals.

With reference to FIG. I, an example downlink frame 100, which istransmitted from a serving base station (BS) in an LTE compliant system,is illustrated. As is shown, the frame 100 (which is 10 milliseconds inlength) includes ten subframes (each of which are 1 millisecond inlength). Each of the subframes begins with a DLRS that includes a PCFICH(labeled ‘DLRS/PCFICH’). In the illustrated example, a DL subframeincludes two slots, each of which include seven long blocks (LBs) whichencode a symbol. It should be appreciated that the techniques disclosedherein are broadly applicable to UL subframes that employ more or lessthan the illustrated number of LBs. With reference to Slot0, a 1^(st)SSCH is assigned to LB 6 and a PSCH is assigned to LB 7. With referenceto Slot11, a 2^(nd) SSCH is assigned to LB 6 and the PSCH is alsoassigned to LB 7. With reference to Sloth, a PBCH is assigned to LB 1(labeled ‘DLRS/PBCH’) and LBs 2-4. While none of the SSCH, PBCH, orPCFICH cover an entire system bandwidth, the channels provide additionalinformation that can be utilized in conjunction with a DLRS to improvechannel estimation and tracking.

With reference to FIG. 2, an example conventional channel estimationapproach 200 is illustrated. As previously noted, wireless devices inconventional communication systems have performed channel estimationbased on a pilot signal. The channel estimation has then be utilized toperform PBCH detection 206, PCFICH detection 208, and other channeldecoding 210. It should be noted that SCH detection 202 is performedwithout the benefit of channel estimation 204. In a typical LTEcompliant system that is configured according to the prior art, thechannel estimation 204 is done in parallel with the SCH detection 202.

With reference to FIG. 3, an example channel estimation process 300,according to the present disclosure, is illustrated. The process 300,similar to the approach 200, includes an SCH detection block 302 thatperforms SCH detection (decoding), without the benefit of channelestimation. However, unlike the approach 200, the process 300 performsan SCH signal reconstruction (using an SCH signal reconstruction block310, which encodes a decoded SCH signal provided by the block 302). Achannel estimation block 318 then performs a channel estimation based ona reference signal (pilot signal) and a reconstructed SCH signal(provided by the SCH signal reconstruction block 310). The channelestimation is then utilized (by a PBCH detection block 304) to performPBCH detection, utilized (by a PCFICH detection block 306) to performPCFICH detection, and utilized (using other channel decoding block 308)to perform other channel decoding.

According to one aspect of the present disclosure, when a PBCH signalpasses a CRC, PBCH signal reconstruction is initiated by a PBCH signalreconstruction block 314 (which encodes a decoded PBCH signal providedby the block 304), which provides a reconstructed PBCH signal to thechannel estimation block 318. The channel estimation may then beperformed based on the reference signal, the reconstructed SCH signal(provided by the SCH signal reconstruction block 310), and thereconstructed PBCH signal (provided by the PBCH signal reconstructionblock 314).

Similarly, following PCFICH detection (by the PCFICH detection block306, which decodes the PCFICH signal), a PCFICH reconstruction block 316provides a reconstructed PCFICH signal (i.e., encodes a decoded PCFICHsignal) to the channel estimation block 318. Channel estimation may thenbe performed by the channel estimation block 318 based on the referencesignal, the reconstructed SCH signal (provided by the SCH signalreconstruction block 310), the reconstructed PBCH signal (provided bythe PBCH signal reconstruction block 314), and the reconstructed PCFICHsignal (provided by the PCFICH reconstruction block 316). When the PBCHdoes not pass a CRC, PBCH signal reconstruction is not initiated and areconstructed PBCH signal is not provided to the channel estimationblock. It should be appreciated that one or more of the SCH, PBCH, andPCFICH signals may be utilized in conjunction with an RS to provide animproved channel estimation.

Turning to FIG. 4, a channel estimation and tracking process 400, thatmay be employed in a SS of a wireless communication system, is depicted.The process 400, which may be utilized to improve channel estimation andtracking, is initiated at block 402, at which point control transfers toblock 404. In block 404, a receiver of an SS receives an RS and one ormore non-error propagation physical channel signal. Next, in block 406,a processor performs channel estimation (and/or tracking) based on theRS and at least one of the one or more non-error propagation physicalchannel signals. Next, in block 408, the process 400 terminates andcontrol returns to a calling routine.

With reference to FIG. 5, an example wireless communication system 500is depicted that includes a plurality of subscriber stations or wirelesscommunication devices 502, e.g., hand-held computers, personal digitalassistants (PDAs), cellular telephones, etc., that may perform channelestimation and tracking according to one or more embodiments of thepresent disclosure. In general, the devices 502 include a processor 508(e.g., a digital signal processor (DSP)), a transceiver 506, and one ormore input/output devices 504 (e.g., a camera, a keypad, display, etc.),among other components not shown in FIG. 5. As is noted above, accordingto various embodiments of the present disclosure, a technique isdisclosed that generally improves channel estimation and tracking. Thedevices 502 communicate with a base station controller (BSC) 512 of abase station subsystem (BSS) 510, via one or more base transceiverstations (BTS) 514, to receive or transmit voice and/or data and toreceive control signals. In general, the BSC 512 may also be configuredto choose a modulation and coding scheme (MCS) for each of the devices502, based on channel conditions.

The BSC 512 is also in communication with a packet control unit (PCU)516, which is in communication with a serving general packet radioservice (GPRS) support node (SGSN) 522. The SGSN 522 is in communicationwith a gateway GPRS support node (GGSN) 524, both of which are includedwithin a GPRS core network 520. The GGSN 524 provides access tocomputer(s) 526 coupled to Internet/intranet 528. In this manner, thedevices 502 may receive data from and/or transmit data to computerscoupled to the Internet/intranet 528. For example, when the devices 502include a camera, images may be transferred to a computer 526 coupled tothe internet/intranet 528 or to another one of the devices 502. The BSC512 is also in communication with a mobile switching center/visitorlocation register (MSC/VLR) 534, which is in communication with a homelocation register (HLR), an authentication center (AUC), and anequipment identity register (MR) 532. In a typical implementation, theMSC/VLR 534 and the HLR, AUC, and EIR 532 are located within a networkand switching subsystem (NSS) 530, which performs various functions forthe system 500. The SGSN 522 may communicate directly with the MR, AUC,and EIR 532. As is also shown, the MSC/VLR 534 is in communication witha public switched telephone network (PSTN) 542, which facilitatescommunication between wireless devices 502 and land telephone(s) 540.

As used herein, a software system can include one or more objects,agents, threads, subroutines, separate software applications, two ormore lines of code or other suitable software structures operating inone or more separate software applications, on one or more differentprocessors, or other suitable software architectures.

As will be appreciated, the processes in preferred embodiments of thepresent invention may be implemented using any combination of computerprogramming software, firmware or hardware. As a preparatory step topracticing the invention in software, the computer programming code(whether software or firmware) according to a preferred embodiment willtypically be stored in one or more machine readable storage mediums suchas fixed (hard) drives, diskettes, optical disks, magnetic tape,semiconductor memories such as read-only memories (ROMs), programmableROMs (PROMs), etc., thereby making an article of manufacture inaccordance with the invention. The article of manufacture containing thecomputer programming code is used by either executing the code directlyfrom the storage device, by copying the code from the storage deviceinto another storage device such as a hard disk, random access memory(RAM), etc., or by transmitting the code for remote execution. Themethod form of the invention may be practiced by combining one or moremachine-readable storage devices containing the code according to thepresent invention with appropriate standard computer hardware to executethe code contained therein. An apparatus for practicing the inventioncould be one or more computers and storage systems containing or havingnetwork access to computer program(s) coded in accordance with theinvention.

Although the invention is described herein with reference to specificembodiments, various modifications and changes can be made withoutdeparting from the scope of the present invention as set forth in theclaims below. For example, many of the techniques disclosed herein arebroadly applicable to a wide variety of wireless communication systems.Accordingly, the specification and figures are to be regarded in anillustrative rather than a restrictive sense, and all such modificationsare intended to be included with the scope of the present invention. Anybenefits, advantages, or solution to problems that are described hereinwith regard to specific embodiments are not intended to be construed asa critical, required, or essential feature or element of any or all theclaims.

Unless stated otherwise, terms such as “first” and “second” are used toarbitrarily distinguish between the elements such terms describe. Thus,these terms are not necessarily intended to indicate temporal or otherprioritization of such elements.

1.-20. (canceled)
 21. A method for performing channel estimation in awireless communication device: receiving one or more reference signalsand one or more physical channel signals at a wireless communicationdevice; reconstructing at least one of the received one or more physicalchannel signals to produce at least one reconstructed signal; andperforming channel estimation based on at least one of the one or morereference signals and the at least at least one reconstructed signal.22. The method of claim 21, wherein the reconstructing is performed whenthe at least one of the received one or more physical channel signalspasses a cyclic redundancy check.
 23. The method of claim 21, whereinthe one or more reference signals are received over a channel other thanthe one or more physical channel signals.
 24. The method of claim 21,further comprising performing tracking based on at least one of the oneor more reference signals and the at least at least one reconstructedsignal.
 25. The method of claim 21, wherein the at least onereconstructed signal is configured to provide a plurality of trainingpilots for the channel estimation.
 26. The method of claim 21, whereinreceiving the one a more reference signals and the one or more physicalchannel signals comprises receiving a subframe comprising a plurality ofOrthogonal Frequency-Division Multiplexing (OFDM) symbols.
 27. Themethod of claim 26, wherein the physical channel signals comprisenon-error propagation physical channel signals.
 28. The method of claim21, wherein the physical channel signals comprise non-error propagationphysical channel signals.
 29. The method of claim 28, wherein the one ormore non-error propagation physical channel signals do not contain dataspecific to the wireless communication device.
 30. A wireless device,comprising: a processor; one or more wireless interfaces in datacommunication with the processor; and logic in data communication withthe processor and the one or more wireless interfaces, the logicconfigured to: receive at least one reference signal and at least onephysical channel signal; and perform channel estimation based at leastin part on the received at least one reference signal and the at leastone physical channel signal; wherein the at least one reference signalis received over a channel other than the at least one physical channelsignal.
 31. The wireless device of claim 30, wherein the logic isfurther configured to perform tracking based at least in part on thereceived at least one reference signal and the at least one physicalchannel signal.
 32. The wireless device of claim 30, wherein the channelestimation is performed when the received at least one physical channelsignals passes a cyclic redundancy check.
 33. The wireless device ofclaim 30, wherein the at least one physical channel signal comprises anon-error propagation signal configured to provide a plurality oftraining pilots for the channel estimation.
 34. The wireless device ofclaim 30, wherein the channel estimation performed based at least inpart on the received at least one reference signal and the at least onephysical channel signal has reduced error compared to a channelestimation performed using only the received at least one referencesignal.
 35. The wireless device of claim 30, wherein the receiving ofthe at least one of the reference signal and the at least one physicalchannel signal comprises receiving a subframe comprising a plurality ofOrthogonal Frequency-Division Multiplexing (OFDM) symbols.
 36. Thewireless device of claim 30, wherein the at least one physical channelsignal comprises a non-error propagation signal selected from a groupconsisting of: (i) a synchronization channel signal, (ii) a physicalbroadcast channel signal, and (iii) a physical control format indicationchannel signal.
 37. A non-transitory computer readable apparatus for usein a wireless device, the computer readable apparatus comprising aplurality of instruction configured to, when executed by a digitalprocessor: receive one or more reference signals and one or morenon-error propagation physical channel signals at a wireless device;reconstruct at least one of the received one or more non-errorpropagation physical channel signals to produce at least onereconstructed signal; and perform channel estimation based on at leastone of the one or more reference signals and the at least at least onereconstructed signal.
 38. A method for performing channel estimation ina wireless communication system, the method comprising: receiving at afirst wireless device one or more references signals and one or morenon-error propagation physical channel signals transmitted from a secondwireless device; performing channel estimation based on the received oneor more references signals and the one or more non-error propagationphysical channel signals; and transmitting from the first wirelessdevice information relating to the performed channel estimation to thesecond wireless device.
 39. The method of claim 38, wherein theinformation relating to the performed channel estimation is configuredto adjust one or more parameters of the second wireless device.
 40. Themethod of claim 38, wherein the channel estimation is performed afterthe received at least one non-error propagation physical channel signalspasses a cyclic redundancy check.
 41. The method of claim 38, furthercomprising: performing tracking based on the received one or morereferences signals and the one or more non-error propagation physicalchannel signals.
 42. The method of claim 38, wherein the one or morereference signals are received over a channel other than the one or morenon-error propagation physical channel signals.